The reaction of an alkylaromatic with an olefinic compound promoted by an alkali metal is a well-known process for adding the olefinic compound to the alkylaromatic compound. In these reactions, the alkylaromatic must have at least one benzylic hydrogen atom and, in the presence of a zero-valent alkali metal, a new covalent bond is formed between the carbon atom bearing the benzylic hydrogen atom and the olefinic compound. The overall process is exemplified by the following reaction showing the reaction of ethylene with toluene to form n-propylbenzene. ##STR1## This type of reaction is commonly referred to as an alkylaromatic side-chain alkylation reaction. This reaction has also been described as a nucleophilic addition of the carbanion of the alkylaromatic to the olefinic compound. Other examples of this alkylation reaction include the reaction of toluene with styrene to form 1,3-diphenylpropane, and the reaction of propene with toluene to form isobutylbenzene (2-methyl-3-phenylpropane). As disclosed in U.S. Pat. Nos. 3,228,831 and 3,385,887, isobutylbenzene is a starting material for the preparation of Ibuprofen {.alpha.-methyl-4-(2-methylpropyl) benzeneacetic acid or 2-(4-isobutylphenyl) propionic acid}. Ibuprofen is a major, over-the-counter analgesic and anti-inflammatory pharmaceutical.
More recently, there has been an interest in preparing alkyl-substituted naphthalene compounds suitable for oxidation to naphthalenedicarboxylic acids. For example, 2,6-dimethylnaphthalene can be oxidized to 2,6-naphthalenedicarboxylic acid. 2,6-Naphthalenedicarboxylic acid is a monomer useful for preparing high performance polyester compositions. Although dimethylnaphthalenes such as 2,6-dimethylnaphthalene can be isolated from certain refinery streams, the concentration of the desired 2,6-dimethylnaphthalene in such streams is typically low and it is usually difficult to isolate the 2,6-dimethylnaphthalene in suitably large quantities. Consequently, synthetic procedures are useful for preparing dimethylnaphthalenes, and particularly 2,6-dimethylnaphthalene, starting from readily available materials. One such procedure, commonly referred to as the "Alkenylation Process", comprises reacting o-xylene with butadiene in the presence of a zero-valent alkali metal to form 5-ortho-tolylpentene (5-OTP). The alkali metal-promoted reaction of an alkylaromatic with a conjugated diene such as butadiene to form an olefinically substituted aromatic is referred to as an alkenylation reaction. The 5-OTP is subsequently cyclized to form 1,5-dimethyltetralin (1,5-DMT), the 1,5-DMT is dehydrogenated to 1,5-dimethylnaphthalene (1,5-DMN), and the 1,5-DMN is isomerized to the desired 2,6-dimethylnaphthalene (2,6-DMN). The "alkenylation step" of this overall process is depicted in the following equation: ##STR2##
In order for the overall Alkenylation Process to be commercially successful for preparing 2,6-DMN--particularly because of the number of process steps involved--it is important to obtain a high yield in each step of the Alkenylation Process. Consequently, it is important for the preparation of 5-OTP from o-xylene and butadiene to proceed in high selectivity and high yield. Additionally, it is desirable to use as little alkali metal as possible because the alkali metal is expensive and usually not recyclable, and also because the reduced level of alkali metal is easier to quench. Residual alkali metal in the reaction product is quenched to avoid the possibility of ignition caused by alkali metal that subsequently comes in contact with moist air.
The art needs an improved process for the side-chain alkylation of alkylaromatic compounds with olefinic compounds. The present invention provides such an improved process wherein the alkali metal promoter is ultrasonically irradiated, i.e. sonicated, in order to produce a superior alkali metal promoter.
Methods for alkylating alkylaromatics with olefinic compounds using alkali metal promoters are taught in Pines and Stalick "Base-Catalyzed Reaction of Hydrocarbons and Related Compounds", pp. 240-308, Academic Press, New York, 1977. Processes for alkenylating xylenes with butadiene are disclosed in U.S. Pat. Nos. 3,766,288 and 3,953,535 to Shima et al. and in U.S. Pat. No. 3,244,758 to Eberhardt. A comprehensive discussion of the use of ultrasonic waves in synthetic organic chemistry, including the sonication of alkali metals, is disclosed in Einhorn et al., Synthesis, November 1989, pp. 787-813, c.f. pages 800-801, wherein it is disclosed that sonicated alkali metal can be used to prepare aromatic radical-anions, catalyze such reactions as the Dieckmann and Thorpe-Ziegler cyclizations, and to enolize ketones. These references do not, however, teach the use of sonication to improve the side-chain alkylation of an alkylaromatic with an olefinic compound.